The terms "rigid articles" or "rigid materials", as used herein, means tubes, sheets, plates, boards and other similar materials which are obtained by extrusion, together with various articles manufactured from these extruded materials, for example, articles made by thermoforming extruded sheets.
The art is replete with methods for making articles from foamed polystyrene. The foamed polystyrene articles have good rigidity and shape retention, but have the disadvantages of being fragile and having poor chemical resistance, oil resistance and thermal stability. Generally, polypropylene has excellent heat resistance, oil resistance and chemical resistance, high mechanical strength, higher flexibility or better elasticity than polystyrene, and higher rigidity than polyethylene. Accordingly, various atttempts have been made to obtain expanded or foamed articles by using polypropylene having these excellent properties.
U.S. Pat. No. 4,442,233 to Lohmar et al, relates to a process for producing cross-linked polypropylene with from about 2 to about 20 weight percent, based upon the weight of the polypropylene of polybutadiene with a molecular weight of from about 500 to about 10,000 under conditions sufficient to effect cross-linking, for example, through the use of cross-linking agents and/or irradiation. In addition, the process disclosed therein may be utilized to produce cross-linked and foamed polypropylenes.
U.S. Pat. No. 4,379,859 to Hirosawa et al, disclosed substantially non-crosslinked pre-foamed particles of a polypropylene resin, characterized by the fact that they are composed of an ethylene/propylene random copolymer or a mixture of an ethylene/propylene random copolymer with low density polyethylene and/or an ethylene/vinyl acetate copolymer as a base resin; and a process for producing pre-foamed particles of a polypropylene resin, which comprises dispersing substantially non-crosslinked particles of the polypropylene resin composed of an ethylene/propylene random copolymer or a mixture of an ethylene/propylene random copolymer with low-density polyethylene and/or an ethylene/vinyl acetate copolymer and a volatile blowing agent in water in the presence of a dispersing agent within a closed vessel; heating the dispersion to a temperature above a temperature at which the resin particles soften, thereby to impregnate the blowing agent in the resin particles; and while maintaining the pressure of the inside of the vessel higher than the vapor pressure of the blowing agent, opening one of the vessel to release the resin prticles and water simultaneously into an atmosphere kept at a lower pressure than the inside of the vessel.
U.S. Pat. No. 4,303,757 to Kajimura et al, discloses a process for producing expandable thermoplastic resin beads which comprises suspending in an aqueous medium 20 to 70% by weight of polypropylene resin particles and 30 to 80% by weight of a vinyl aromatic monomer such as styrene, polymerizing the vinyl aromatic monomer in the presence of a polymerization catalyst to graft the vinyl aromatic monomer onto the backbone of polypropylene and, optionally, adding a cross-linking agent, to form graft-copolymerized thermoplastic resin beads, and introducing a blowing agent into the thermoplastic resin beads. The resulting resin beads have good foamability and molding fusability, and a foamed shaped article having good thermal stability can be prepared from these beads.
U.S. Pat. No. 4,298,706 to Ueno et al, described foams of polypropylene resin composition having remarkably improved extrusion foaming characteristics prepared by a non-crosslinking extrusion foaming process comprising mixing and kneading polypropylene resin and 1,2-polybutadiene resin so that 1,2-polybutadiene resin reacts under the influence of the heat and shearing stress in the extruder, and extruding thereafter.
As can be seen from the above, the art does not provide a method to obtain articles, having good physical characteristics, from foamed or expanded polypropylene without the addition of significant amounts of polystyrene, polybutadiene and the like. Even with the utilization of highly specialized and specific processes, blowing agents and other additives, the heretofore known conventional processes for forming expanded polypropylene produce cells of uneven structure and give surface irregularities and poor appearance to the resulting foam, thereby yielding products of reduced commercial value.
Additionally, previously known methods for producing expanded polypropylene articles have certain disadvantages. For example, such articles have poor mechanical strength, and more particularly poor flexural strength. Moreover, the specific gravity or density of such articles is not always homogeneous. Thus, the articles may contain density variations that may reach about 10%.